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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Subject: Re: new_line in a put_line
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Date: Wed, 4 Dec 2002 15:23:14 GMT
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Date: 2002-12-04T15:23:14+00:00
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Dmitry A. Kazakov <mailbox@dmitry-kazakov.de> writes:

> On 04 Dec 2002 10:31:15 +0100, Fraser Wilson
> <newsfraser@blancolioni.org> wrote:
> 
> >Dmitry A. Kazakov <mailbox@dmitry-kazakov.de> writes:
> >
> >> Just do not do I/O from them. Define a mutex as a protected object and
> >> a spin lock as a controlled type (less troubles with releasing a
> >> mutex):
> >
> >[..]
> >
> >>    procedure Put_Line (Text : String) is
> >>       Get_It : Lock (Write_Mutex'Access);
> >>    begin
> >>       Ada.Text_IO.Put_Line (Text);
> >>    end Put_Line;
> >
> >Something inside me rebels at using side-effects from a declaration
> >like that. 
> 
> Right, right. It is also my opinion [there was a thread regarding this
> subject in c.l.a] that using unused (:-)) objects probably indicates a
> design problem. A task-based design might be better.

There is nothing wrong with declaring a limited controlled object that
is used only for its Initialize/Finalize effects.  It's a common idiom
in Ada (and also in C++).

It's the *only* way in Ada to safely ensure that resources get cleaned
up -- so telling people not to use it is bad advice, IMHO.

You could reasonably argue that Ada ought to have a better syntax for
this -- one that does not involve creating dummy types.  In fact, such a
syntax was proposed as part of Ada 9X, but was rejected on the grounds
that limited controlled types provide all the necessary functionality.

A task-based design would work just fine, too.

- Bob